The present invention is concerned with an expansion anchor, and more particularly with an expansion anchor which is especially suitable for use in concrete structures.
From my aforementioned U.S. Pat. No. 3,837,257, it is known to provide an expansion anchor that is particularly suitable for use in concrete structures and which utilizes an expansion anchor sleeve in form of a wire spring which is inserted into an expansion anchor hole in the respective structure, and into the leading end of which an expander member of generally conical configuration is drawn by a screw or bolt which is inserted through the sleeve from the trailing end thereof and engages the expander member. The advantage of this construction is that when the sleeve is being expanded, the developing expansion forces will act essentially in radial direction and will anchor the device in the surrounding concrete structure with the least possible damage to the concrete itself. It is well known that concrete wnen subjected to high localized pressures tends to break away, and by the use of the wire-spring anchor sleeve I have eliminated the kerf action which is the result of such localized pressures in prior-art constructions and which leads to the breaking-away of the concrete.
However, although my prior expansion anchor eliminates the difficulties which are heretofore attendent upon the expansion of an expansion anchor in concrete, it does not overcome a second problem which develops when external forces act upon the expander member, for instance via the expander bolt, in a sense tending to draw it further into the sleeve towards the outer end of the expansion anchor hole, beyond the degree of insertion which the expander member had already reached during the initial expansion of the sleeve. If this takes place, an abutment provided on the expander member prevents it from being fully drawn into the sleeve, and ultimately through the same and out of the sleeve; however, the engagement of the abutment with the sleeve, and the consequent further expansion of the same results in the development of very high localized forces which act via the expanding convolutions of the sleeve upon the surrounding concrete. When this takes place, in the region of the inner end of the expansion anchor hole, it will in all likelihood not cause sufficient construction of the surrounding concrete to loosen the expansion anchor. However, should the anchor slip outwardly, or should the inner end of the sleeve with the expander member for some reason be located closer to the outer end of the expansion anchor hole, then the danger of breaking away of the concrete becomes the greater, the closer the inner end of the sleeve and the expander member are located to the outer end of the expansion anchor hole. Once the concrete breaks away the expansion anchor will, of course, be freely extractable from the hole.